Generally the austenitic steel which is expressed by 18% Cr-8% Ni (STS 304) is superior in the formability, corrosion resistance and weldability compared with the ferritic stainless steel, and therefore, the austenitic stainless steel is widely used for press-forming purposes.
However, the austenitic stainless steel contains a large amount of the expensive element Ni, and therefore, its cost is very high.
Therefore, attempts have been made to manufacture a high formability stainless steel with the content of Ni reduced.
One of such attempts is Japanese Patent Publication No. Sho-43-8343 in which the proposed stainless steel contains less than 0.15% of C, 5.5-8.0% of Ni, 16-19% of Cr, 0.5-3.5% of Cu, and 0.04-0.1% of N.
However, in the case of the above stainless steel, the ingredient ranges are too wide, and therefore, the formability and other properties show much deviations. Further, the contents of C and N are too high, and therefore, the season cracking resistance is unsatisfactory. Particularly, the addition of Cu aggravates the hot workability.
Further, another proposal is disclosed in Japanese Patent Laid-open No. Sho-52-119414 and Sho-54-128919, in which Cu is added, and the content of Mn is raised by 2% in place of Ni. In this case, The content of Mn is too high, and therefore, the high temperature oxidation resistance is lowered, so that surface defects may occur due to a high temperature oxidation during the hot rolling of the slab. Further, when manufacturing a bright annealing sheet, blue color may occur during the bright annealing.
Still another attempt is seen in Japanese Patent Publication No. Sho-59-33663, in which the stainless steel containing Cu is made to contain less than 1% of an ingredient selected from a group consisting of Nb, Ti and Ta, so that the crystalline grains would become fine, thereby improving the formability of the stainless steel.
In this case, however, the content of C is too high, and therefore, the season cracking resistance is lowered.
Still another attempt is seen in Japanese Patent Laid-open No. Sho-54-13811, in which 0.005-1.0% of Nb is added to a steel containing extremely low levels of C and N. Thus the crystalline grains are made fine, and the austenitic phase is reinforced, so that the stretch ability would be improved.
In this case, however, due to the extremely low level of C and N, the refining work lowers the productivity, and the austenite equivalence is low, with the result that the content of the delta-ferrite is increased, thereby aggravating the hot workability.
Still another attempt is seen in Japanese Patent Laid-open No. Hei-1-92342 and German Patent Publication No. 302975. In the case of the former, a steel containing Cu is made to contain tiny amounts of Ti and B, and less than 50 ppm of oxygen, and less than 0.006% of Ca. Thus the formation of inclusion is inhibited, thereby improving the formability. In the case of the German patent, a steel containing Cu and B is made to contain one element or two selected from the group consisting of Nb, V, Ti and Zr by less than 0.15%. Thus the corrosion resistance, creep strength and the formability are improved. However, in these two inventions, the content of Ni is as high as 8%, and the high content of Ni makes the steel uneconomical.
There is still another attempt disclosed in Japanese Patent Publication No. Sho-55-89568, in which the steel contains 6-9% of Ni, 16-19% of Cr, less than 3% of Cu and 0.5-3.0% of Al, and further contains two elements selected from a group consisting of Nb, Ti, V, Zr and Ta by 0.2-1.0%, thereby improving the formability of the steel. In this case, however, the formation of an inclusion oxide material becomes very high due to the high content of Al, with the result that surface defects such as linear defect, sliver and the like occur on the hot rolled coil.